The present invention relates to a numerically controlled grinding machine which performs external or internal grinding on a workpiece.
Conventionally is known a numerically controlled grinding machine which automatically selects a rough grinding wheel and finish grinding wheel sequentially according to a sequence of grinding operation of the workpiece. In such grinding machine, a workpiece is ground into a predetermined shape by the successive grinding from the rough to finish grinding operations while the workpiece is mounted on a spindle.
In the rough grinding operation with the rough grinding wheel, since the surface accuracy of the machined workpiece is not necessary to be considered, the purpose of using the rough grinding wheel is different from that of a finish grinding wheel and only the better workability or the grinding rates is emphasized. For that reason, a grinding wheel consisting of a single layer of abrasive grains formed by electro-deposition is often used as the rough grinding wheel and in that case from the economical point of view, the rough grinding wheel is thrown away without dressing to eliminate the wear and cracks thereof. Thus, for the rough grinding wheel, even the wheel size is not supervised by the numerical controller.
Now, the rough grinding wheel wears as it is used and its wheel diameter reduces gradually from the initial value. In rough grinding operation by the worn rough grinding wheel, then, the workpiece can not be ground to the theoretical machining sizes designed with the initial wheel diameter, leaving unground portion on the workpiece. As a result, an allowance for machining by the finish grinding wheel in finish grinding operation becomes larger than the theoretical value. Thus, the finish grinding amount to be ground by the finish grinding wheel increases more than necessity, which has the finish grinding wheel excessively over-loaded, so that the surface and size accuracies etc. of the machined workpiece are badly influenced and wheel life of the finish grinding wheel will be shortened.